It is usual for the primary flight control surfaces of an aircraft, for example elevators, rudders, and ailerons, to be actuated by dual hydraulic actuators. Dual actuators can be in the form of two separate hydraulic actuators, although more often are in the form of dual-tandem actuators in which a single piston rod and output member carries two spaced pistons, each piston operating in a respective hydraulic cylinder. The two cylinders of the dual actuator arrangement are supplied separately with hydraulic fluid under pressure from respective discreet pressure sources through the intermediary of respective hydraulic control valves. The capacity of each hydraulic supply and piston and cylinder arrangement is such that the primary flight control surface can be operated by either piston operating alone, and thus the overall system can safely accommodate failure of one or other of the hydraulic supplies, control valves, or piston and cylinder arrangements.
Although the two hydraulic control valves controlling the cylinders of a dual actuator could be completely separate from one another it is essential that the cylinders of the dual actuator are operated in unison, and any lack of synchronisation in the operation of the two valves could result in an undesirable “force fight” as part of the dual actuator tries to perform an operation not being performed by the other part of the actuator. In order to avoid such difficulties a tandem control valve of the kind illustrated in FIG. 1 of the accompanying drawings has been proposed. The tandem valve of FIG. 1 includes an outer valve block 11 containing a valve body, usually referred to as a valve guide 12, slidably receiving a valve spool 13. FIG. 1 also illustrates a dual-tandem actuator 14 having first and second cylinders 15, 16 housing respective pistons 17, 18 carried by a common piston rod 19. One end of the piston rod 19 has a coupling 21 to the primary flight control surface, and the position of the piston rod 19 relative to a fixed datum position is measured by an LVDT (Linear Variable Differential Transformer) 22 or other sensor the output of which is supplied to a control system of the aircraft. In the embodiment shown the cylinders 15, 16 are fixed in position, and the actuator 14 is a double acting actuator in the sense that the piston rod 19 can be driven to the left or the right (in FIG. 1) usually to deploy or retract the associated flight control surface, by admission of hydraulic fluid under pressure to appropriate ends of the cylinders 15, 16. The invention, is applicable also in relation to single acting actuators wherein the actuator is hydraulically driven in a deploy direction and otherwise returned in the retract direction. Moreover rather than the two hydraulic circuits controlling pistons of a dual-tandem actuator they could control respective separate actuators which would thus operate in unison.
Each cylinder 15, 16 has respective flow and return lines 23a, 23b and 24a, and 24b connected to respective first and second control ports 25a, 25b and 26a, 26b of the valve guide 12. Positioned between the first and second control ports 25a, 25b is a supply port 28 connected to a first hydraulic supply line 33 for hydraulic fluid under pressure. Similarly a supply port 29 is disposed between the ports 26a and 26b and is connected to a hydraulic supply line 34 connected to a second source of hydraulic pressure. Outwardly, beyond the second control port 25b, the valve guide 12 has a return port 31a, and a second return port 31b is disposed between the control port 25a and a notional mid-plane 27 of the valve guide 12. The return ports 31a and 31b are connected to a common low pressure return line 35. The valve guide to the right of the plane 27 (the guide of the right-hand control valve of the tandem valve) is similarly provided with first and second return ports 32a, 32b connected to a common low pressure return line 36.
The spool 13 of the tandem valve can be viewed as two integral spools, one for each valve, and each has a centrally disposed gallery 43, 46 which, dependent upon the axial position of the spool, can connect the respective supply ports 28, 29 to one or other of the respective first and second control ports 25a, 25b and 26a, 26b. On opposite sides of each centre gallery 43, 46 the spool is provided with first and second return galleries 44a, 44b and 47a, 47b. 
When supply gallery 43 interconnects supply port 28 and control port 25a then return gallery 44b interconnects return port 31b and control port 25b. Similarly with the spool moved in the opposite direction, to the right, and supply gallery 43 connects supply port 28 and control port 25b then return gallery 44a interconnects return port 31a and control port 25a. Moreover, it will be understood that while gallery 43 interconnects ports 28 and 25a, the supply gallery 46 will be interconnecting supply port 29 with control port 26a and return gallery 47b will be connecting return port 32b with control port 26b. Similarly when supply gallery 46 interconnects supply port 29 with control port 26b then return gallery 47a connects return port 32a and control port 26a. 
It will be recognised therefore that with the tandem valve in the position shown in FIG. 1 the lands of the spool which separate the gallery 43 from the gallery 44a and the gallery 43 from the gallery 44b, are in position closing the control ports 25a and 25b. At the same time the lands at the right hand end of the spool 13 close the control ports 26a, 26b, and both cylinders 15 and 16 have their pistons 17, 18 hydraulically locked in position so that the piston rod 19 is immovable, there being no flow path along the hydraulic lines 23a, 23b and 24a, 24b. 
Movement of the spool 13 to the left from the position shown in FIG. 1 supplies hydraulic fluid under pressure from the supply line 33 to the left-hand side of the piston 17, while opening the line 23b from the right-hand side of the piston to the low pressure return line 35. Simultaneously, and in synchronism therewith, the left-hand side of the piston 18 is exposed to hydraulic pressure from the supply line 34 through the line 24a, and the right-hand side of the piston 18 is connected to the low pressure return line 36 through the line 24b. Thus the piston rod 19 is moved to the right to deploy the primary flight control surface.
Movement of the spool 13 to the right, through the full extent of its travel, reverses the connections to the lines 23a, 23b, and 24a, 24b so that the right-hand sides of the pistons 17, 18 are exposed to hydraulic pressure from the supply lines 33 and 34 while the left-hand sides of the pistons 17, 18 are connected to the low pressure return lines 35 and 36 respectively, thereby driving the piston rod 19 to the left and retracting the associated primary flight control surface.
Each of the ports 25, 26, 28, 29, 31, 32 is defined in the valve guide 12 by a circumferential, rectangular cross-section groove in the outer surface of the valve guide. The grooves are closed so as to define annular galleries by the inner surface of the valve block 11. The wall of the guide 12 has a plurality of radial drillings extending inwardly from the channels defining the various ports, and opening at the inner face of the guide 12 to coact with the galleries of the spool 13. The spool 13 is moved within the guide 12 by the application of pressurised hydraulic fluid to control chambers 38, 39 at opposite ends respectively of the spool 13. Application of pressure to the chamber 38 while venting the chamber 39 moves the spool 13 to the right in FIG. 1 against the action of a return spring 41, while application of control pressure to the chamber 39 while venting the chamber 38 moves the spool to the left against the action of a further return spring 42.
It can be seen that at each side of each annular port defined between the guide 12 and the valve block 11 there is provided an “O”-ring seal which seals the interface of the guide 12 and the valve block 11 to prevent leakage along that interface. Furthermore, the large central land of the spool 13, between the galleries 44b and 47a, is provided with an ‘O’-ring seal sealing the sliding interface of the spool 13 and the guide 12 to prevent leakage between the galleries 44b and 47a along that interface. It will be recognised that it is most important, for safety considerations, to preserve the isolation of the two hydraulic systems and in normal operation the arrangement, including the ‘O’-ring seals, achieves such isolation of the systems from one another. However, it has been recognised that there is a possible fault condition of the tandem valve of FIG. 1 in which the spool 13 fractures along a line joining the gallery 44b and the gallery 47a. Such a fracture line is shown at 50 in FIG. 1 and could cause leakage from the gallery 44b to the gallery 47a and vice versa leading to cross contamination of the two separate hydraulic systems and possible undesirable pressurisation or loss of pressurisation of a cylinder.
In practice the valve block 11 may have a drain drilling aligned on plane 27 and the spool 13 will have a pair of spaced “O”-ring seals on its central land, the drilling draining any leakage past the seals from the galleries 44b, 47a. Moreover the block 11 may be formed in two halves divided on the plane 27.
It is an object of the present invention to provide a tandem valve wherein such problems are minimised or obviated.